Agreed everything in the diet is wrong (7)
I believe the answer is:
tallied
'agreed' is the definition.
('tally' can be a synonym of 'agree')
'everything in the diet is wrong' is the wordplay.
'everything' becomes 'all' (all things).
'in the' means one lot of letters goes inside another.
'is wrong' indicates an anagram.
'diet' with letters rearranged gives 'tied'.
'all' going within 'tied' is 'TALLIED'.
(Other definitions for tallied that I've seen before include "Agreed; calculated" , "Reckoned debt or score" , "Matched" , "Corresponded - kept score" , "Agreed, corresponded" .)